Low carbon steels and low alloy steels with a yield strength of 235-345 MPa have been used in steel construction structures for a long time in China. At present, the yield strength grade of low alloy steel has been extended to 460 MPa in “Ministry of Construction of the People's Republic of China: Code for design of steel structures” GB 50017-2003. A steel construction pipe with a high strength grade is used to thin the section size of a steel structure and save investment, but a steel structure with a thin section is prone to have the problem of local and overall yield instability, thus limiting the thinning of the section size of a steel structure, resulting in that the advantages of a steel construction material with a high strength grade cannot be fully exploited. Therefore, an urgency for the development of the construction steel with a high strength grade is to solve the problem of yield instability of a steel structure with a thin section. A steel pipe with a thin section and a high strength grade is filled with concrete to form a high-strength steel pipe-concrete structure, where the steel pipe and concrete work together, and which can significantly increase the stiffness thereof, solving the problem of yield stability of the structure. Because a high-strength steel pipe-concrete structure can make full use of the advantages of a high-strength steel material and a high-strength concrete, and can save investment and accelerate the construction schedule, it is a technical growth point having the development potential for expanding the market of construction steels with high strength grades.
Compared with a seamless pipe, a welded pipe has the outstanding features of high production efficiency, good dimension precision, wide range of specifications, low cost and the like, and therefore, is the preferred type for manufacturers and oil field users, and is favoured by the market. The production process of an ERW casing includes: steel-making, continuously casting, hot rolling into a plate roll, shearing and butt welding the plate roll end-to-end, forming into a plate strip, welding on-line, performing heat treatment on the welding seams or performing heat treatment on the whole pipes, processing the pipe, performing delivery inspection and so on. The process step of shearing and butt welding the plate roll end-to-end is the key process step for realising a multi-roll continuous production, which reflects the production efficiency of ERW welded pipes. However, it is required in the butt welding of a plate roll end-to-end that the carbon equivalent in the material must be low, otherwise it would be prone to cause strip breakage of a steel plate, seriously influencing the production efficiency. Furthermore, a high-strength steel pipe-centrifugal concrete member used as a construction pile foundation often bears a great impact load, and therefore it is required that the steel pipe has an excellent strength and toughness and a low yield ratio, and at the same time, it is also required that the welding seams thereof have excellent mechanical properties; consequently, the carbon equivalent Ceq of the material is required to be no greater than 0.4.
The Japanese patent JP 56035749 A proposed a method for an ERW casing, the components of the casing did not contain strengthening elements of Ti, Nb, V, Cr and the like. Although the requirement of a high strength could be satisfied, the Si content reached up to 1% so that grey spots were prone to form at the welding seams during welding, seriously affecting the quality of the welding seams, and the carbon equivalent reached up to 0.8, making the shearing and butt welding impossible.
In the Japanese patents JP 09029460 A, JP 54097523 A, JP 56069354 A and JP 59047364 A, Cu was added, and the requirements of producing a high-strength casing may also be satisfied, but since Cu is substantially insoluble in ferrite at room temperature and is precipitated in the form of ε-Cu or face-centered cubic α-Cu, the strengthened effect of steel has great sensitivity to the cooling rate, therefore making it difficult to stably control the properties of the hot rolled plate in the controlled rolling and controlled cooling process of the production of a hot rolled plate;
The Japanese patent JP 57131346 A provided a welded pipe with resistance to grooving corrosion, wherein, in order to reduce the generation of MnS, and based on controlling the content of S at a low level, more than one of Cu, Ni, Al and Cr, and even harmful elements of As, Sb, Sn, Bi and the like were added, or more than one of Ti, Nb, Zr, V and the like was further added, so that the requirement of a high strength and toughness of 500 MPa of the present invention cannot be achieved at all. The Japanese patents JP 58093855 A and JP 59096244 A both contained an expensive Ni element, causing the alloy cost to be relatively high; and in addition, the Japanese patents JP 57131346 A and JP 58093855 A had relatively high Si contents, making it difficult to obtain excellent properties of the welding seams. Although the steel types involved in the Chinese patents CN 200710038400.1 and CN 200310104863 can achieve the performance requirements of a high strength and a low yield ratio, due to the high content of C and a carbon equivalent of greater than 0.4, they can only meet the requirements of the shearing and butt welding, enabling the production to proceed continuously, but it is difficult to obtain excellent properties of the welding seams, so that the great impact load borne by a pile pipe could not be withstood. The steel type provided in the Chinese patent CN 200310104863 can also satisfy the 500 MPa requirement; however, because the components were too simple, it only achieved the required mechanical properties under a coiling temperature of 430° C.-470° C. Under such a low coiling temperature, it is more difficult to realise the stable control of the properties of a hot rolled plate.